This invention relates to video signal transmission and recording and more particularly to video signal processing, encoding, and decoding systems for video recording. The system includes a novel wide band FM modulator, writing amplifier, and wide-band demodulating circuits and discloses novel electronic circuit implementations of the same using discrete components. The system finds special application to video disc and tape recording, to video recording using a modulated laser beam, as well as to applications in communications systems using an electromagnetic wave carrier.
Many systems have been proposed for the recording of video signals the most familiar application of which is the relatively permanent recording and replay of video and audio TV signals. Also known are various high resolution T.V. systems such as in the fields of medical and industrial video recording. Such systems employ as much as 1,225 line resolution, both vertical and horizontal, and are capable of at least 10 mHz bandwidth video and up to 25 mHz video. The present invention is directly applicable to such systems where the recording media or system can handle such bandwidths.
In the transmission of high resolution television signals, existing commercial broadcast technology requires that the initial video signal, which may be from DC to 25 mHz in bandwidth, be separated and compressed as an AM video signal and FM audio signal on a 6 mHz wide bandwidth (in the United States). Other limitations are presented by the bandwidth characteristics of media used for recording a signal. Typical media include the wellknown magnetic tape consisting of a particulate magnetically impressible material disposed on a plastic strip, a film in which an intermittent motion film may be scanned by a flying spot scanner, an electron beam recorded film digitally encoded films, and laser beam scanned optical systems. Also known are holograms which operate in playback only, being originally recorded by laser beam as a surface deformation on a plastic film.
At the present time, cassette video recorders have become popular utilizing magnetic tape cassettes in which the medium is a typical magnetic tape which is transversely scanned by a system of rotating heads to establish a sufficient head to tape writing speed as to permit a wide frequency band recording. Various electronic systems have been devised for handling the problem of reducing a video signal and audio signal into a form which is capable of being impressed upon the recording media. Typically the bandwidths to which compression is taken have been as low as 11/2 to 2 mHz and FM conversion systems have had to be employed. Since the initial bandwidth of a full high resolution video signal of typical form can be as much as 25 mHz any compression of this bandwidth will result in loss in video fidelity. Even so, with limited bandwidth, in order to record a typical NTSC video signal, i.e., as high as 6 mHz, requires the video tape to move at 6 linear meters a second or faster, and, even at that speed, the wavelengths of the magnetic fields recorded on the tape are so short that submicrometer head gaps are required, and head to tape spacing must be made critically small. In consumer video tape recording such linear speeds consume too much tape so that helical scan systems have been developed in which the tape spirals around a revolving drum to which the tape heads are mounted. This allows a tape head traversing a circular path with the same radius as the drum to scan diagonal tracks on the tape at a speed high enough to record such a video signal. Even so, consumer VTR's are not capable of direct recording of a composite video signal and color subcarrier. These are, therefore, disassembled into a baseband luminance signal and a subcarrier chrominiance signal before being recorded. Even so, a nearly 17 octave frequency range is required to capture the entire luminance signal. This is usually solved by employing narrow band FM techniques. Numerous problems are encountered with these systems. There is a need, therefore, for a new and improved system for signal processing of high resolution composite video signals which will enable the same to be recorded without extensive signal modification and with greater fidelity.
In certain high resolution systems, the foregoing limitations in actual recording are overcome using magnetic disc recording of much higher bandwidth. The discs may be in the form of metal film plated on the plastic substrate disc. Other industrial video systems are known having at least 10 mHz bandwidth and several development systems are proposed having bandwidths up to 25 mHz including proposals for video signal transmission by satellite. In such applications there is a need for a wideband, high resolution signal processing system having commensurate capabilities. In general, the capability which is sought is to provide high resolution television viewing having 35 mm film quality and projectable on large screen. For the present, a 10 mHz bandwidth is the usual limitation and is imposed primarily by the magnetic media characteristics that are used in conventional technology. If, as will occur, the media magnetic characteristics are improved, then the electronics disclosed herein will be capable of operating at the new higher bandwidths without conceptual or configural changes.